1. First, I start out with an image that I've sized down, then copied and pasted onto a blank canvas. I won't go into how I colored it since that's a bit off-topic.
2. Then I choose a texture/background that I like or make a gradient depending on the image and what I have on hand, and open it in a new tab. This time I chose to make a gradient background since I didn't have a texture that quite matched this image.
3. Next, I copy and past the gradient background ONTO the image. Then I go to Layer > Layer Mask > Hide All. Make sure that the layer with the background is selected when you do this step. In this case, layer 2. Also, there's a little 4-cornered box-type thing that should be around the mask. Make sure it's around the mask, and not the image itself. You can see what I'm talking about if you look at the layers tab below.
4. Now you select the brush tool.
5. The next step is to pick the brush size. Depending on the size of the image, I always go with a rather large brush to start out with, and then I decrease the size as I get closer and closer to Serena's body. First off, I'm going to go with about a 64 px with 51% hardness. with hardness, the lower the percentage is, the softer the brush is. If you go at 100% it's a very hard, clean cut but you have a better chance at cutting things out that you don't want cut out. At 1% hardness, you won't get a clean, crisp cut at all. You might want to start out low to begin with, until you get the hang of masks. That's what I did, and then moved up once I got comfortable. I still don't work with brushes at 100% hardness, it's a little too severe for me. I like to go right down the middle.
6. Now that I've got the brush size picked out, I start brushing over the parts I want cut out. Now, I've gotten as close to Serena as I feel comfortable with this size brush.
7. Now, I want to go down to about a 19px brush, and slowly take out the rest of whatever I don't want. If i mess up, I just go to edit > undo. Simple. I usually magnify the image when I get down to the nitty gritty like this. On an icon, usually 300-400% but on a large image like this 200% will usually do it. when I get to Serena's elbow, I go down to about a 9px so that I can get into that tight corner. And voila, finished product!
8. Here's a cap of the layer's toolbar up close. Now, you can see that on the mask, everything in white is what I took out, while the black is everything I left in.